Gain-control system for seismic amplifiers



Sep., 24, E946. E. J. sHlMEK ETAL Zym GAIN-CONTROL SYSTEM FOR SEISMIC AMPLIFIERS l Filed April 14, 1945 l 2 Sheets-Sheet l.

Sep@ 24g, m46.;r

rw) l E. J. SHIMEK Erm GAIN-CONTROL SYSTEM FORv SEISMIC AMPLIFIERS Filed April 14, i945 i vAff/4 BY Patented Sept. 2li, i945 its s 'it GAN-CNTROL SYSTEM FOR SEISMIC AMPLIFIERS Edwin J. Shimek and William B. Hemphill, Dallas,

Tex.,

assignors, by mesne assignments, to

Socony-Vacuum Gil Company, Incorporated, New York, N. Y., a corporation of New York 9 Claims.

This invention relates to electric seismographs, l

more particularly to a method and apparatus for controlling the gain in amplification through the period of time during which seismic waves created in the earths surface are being detected and amplied, and has for an object the provision of a system in which interference with recordation by spurious signals is minimized.

In seismic prospecting systems a charge of dynamite, located in a shot hole, is detonated to produce seismic waves which are reiiected from interfaces loca-ted below the earths surface. These reflected waves are attenuated by their travel through the underground strata. The deeper the interface, the less is the amplitude of the reflected waves although, of course, the particular character of the interface or of the adjoining subsurface strata also plays a part in determining the amplitude of the waves. Though the reflected waves are of primary interest, the direct traveling waves are also received by the wave detector or geophone. It is desirable to utilize the instant of iirst arrival of the direct traveling waves, of large amplitude, for certain calculations. More specifically, a seismogram consists of the recorded waves together with a series of timing lines. These seismograms are interpreted in terms of time intervals and in terms of the patterns of the waves themselves. Each reflection presents its own typical pattern and from these data the depth of the underlying interfaces or substrata may be calculated in manner understood by those skilled in the art.

The times of arrival of the direct traveling waves at each of the several detectors or geophones, generally known as the spread, are determined by the first movements of the respective oscillographic elements. In other words, .the galvanometer or oscillographic elements should be at standstill prior to the receipt of the rst of the direct traveling waves. The time of arrival of such iirst wave is ascertained by the beginning of the movement of the oscillographic element, or the light beam, from its neutral position.

In order to increase the accuracy with which the initial movement of the galvanometer may be ascertained, it has been custo-mary to have the amplifierl set at maximum gain. This produces a sharp break or movement and minimizes the need to estimate the precise instant at which the osciilographic elements start to move. It has been found that any spurious noise or signals which might occur just prior to the instant of arrival oi the direct traveling waves may so blend with the signals produced by the direct traveling waves as to give erroneous records of the First breaks.

In accordance with the present invention, the eifect of spurious signals has been to large degree eliminated by providing a system in which the amplifier is preset for an intermediate sensitivity or amplification. By the initial reduction in the amplification or the sensitivity of the recording system, the elect of spurious signals is to large degree eliminated without losing the benefit of relatively high gain or amplification of the rst of the direct traveling waves. Immediately upon recordation of the rst break, the sensitivity of the ampliiier is quickly reduced to a predetermined minimum value and then increased to a maximum value as a function of time, whereby the recorded amplitudes of the reflected Iwaves are maintained substantially constant.

Further in accordance with the invention, an automatic volume control is provided which is effective over a portion of the period during which expansion of the amplification is taking place.

For a more detailed description of the invention and for further objects and advantages thereof, reference should be had to the following detailed description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a wiring diagram of a typical embodiment of the invention;

Fig. 2 is a graph illustrating the manner in which lthe gain of the amplifier is controlled with respect to time; and

Fig. 3 is a graph illustrating the manner in which a predetermined output voltage, from a xed time constant network, is reached in shorter times, for greater input voltages.

Referring to the drawings, the invention in one form has been shown as applied to the control of an amplifier 5 having an output transformer 6 connected to suitable recording apparatus as indicated by 4the Recorder l. One stage, or tube 8, of the amplifier has been illustrated as a pentode type of tube provided with an input circuit, including the secondary winding of a transformer the primary winding of which is connected to a seismic wave detector in `the form of a geophone lil. Generally there is a separate amplier, of more or less conventional desi-gn, for each ofV the several detectors or geophones of the spread. Each amplifier may include additional stages such as valves or tubes l l and l2, with filters i3 and ifi interposed therebetween. The several amplifiers may be simultaneously controlled from a master controller M in the same manner as will now be described for the amplifier 5.

The gain of one or more tubes, such as tubes 8 and I I, is controlled by the application o1" a suitable negative bias on the suppressor grids 3a. and I Ia. This gain-controlling bias is established between the point d, connected to grid Sa, and the point e, at ground potential, of the controller which includes an electric valve or amplier tube I5, sho-wn as of the pentode type, and a second electric valve or triode tube I. The output from the ampliiier 5 is applied by capacitor I1 and conductol i8 to the control grid of vthe pent-ode I5. This control grid is included in a circuit for negatively biasing it, with reference to the cathode, for operation in manner such that some anode current will ow in the absence of signals applied from the ampliier 5. This biasing means im cludes a resistor I 9 and the right-hand portion of the battery illustrated as the source of anode potential.

As shown, the screen grid of the pentode I5 is connected by way of conductor 2t and the contact 2l of a high speed relay 22 to the point 23 on the battery, for application of a substantially normal screen grid voltage thereto. The suppressor grid is connected in conventional manner to the cathode. The current liows in the anode circuit from B-lof the battery by Way of conductor 24, resistor 26, conductor 2l, from anode to cathode of pentode I5, and by conductor 23 to the point on the battery marked B-.

This flow of current through the resistor 2B produces an IR drop or potential diierence between the points a and c of the circuit.V

Current also flows through a lter network comprising capacitors 3D and 3| and a resistor 32. It will be observed the triode I6 has its cathode connected to the output side of the filter -32 while its anode is connected to ground G and to the positive or B-lside of the battery. The control grid of the triode I6 is connected through the contact of a relay 36, and by conductor 31 to a point 33 on the battery which is less positive than the connection to the battery of the anode of triode I6.

It is to be further observed the cathode of the triode I6 is connected through the resistor 32 and the pentode I 5 to B of the battery, a point more negative (or less positive) than the control grid thereof. Thus, as far as triode I6 is concerned, conditions are established for conduction of current. Therefore, there will be current flow through the triode IE.

The voltage applied to the suppressor grid 8a, of the amplier `tube 8, will be the voltage between the point d and ground G of the controller. This voltage (Ede may be expressed by the following equation:

EdeZEac-IRSZ where Eat is `the voltage across the resistor 25, R32 is the resistance of the resistor 32, and I is the current owing through resistor 32,

Initially the voltage Ede applied to the suppressor grid, or gain-controlling bias, has a magnitude such as to produce pre-suppression of the amplication; that is, this gain-controlling bias reduces the gain of the amplifier 5 to an intermediate value somewhat below the maximum gain thereof.

In accordance with the invention, this gaincontrolling bias which produces the pre-suppression or intermediate value of the amplication, is substantially independent of the magnitude of signals applied to the control grid of the pentode I5. For example, if spurious signals are applied 4 by the geophone I0 or are induced in the circuits by stray elds, or should they arise from any other source, the output of the pentode I5 will increase. For an increased output there will be a greater potential difference across the resistor 26 which will in turn tend to increase the current through resistor 32 in the direction from -d to a, this circuit including the triode I6. The

ltube, I5 for the operating condition thus far described. Y The grid voltage or bias Ede may be applied to suppressor grid IIa of tube II as well as to grid 8a; or the grid IIa may be connected in conventional manner, or to other gain-controlling means.

The magnitude of the initial gain-controlling bias may be varied or selected by adjustment of the dependent variables, the resistance values of the resistors 26 and 32, and theoperating point on the characteristic curve of the triode I6. In other Words, the voltage or bias between the points d and e may be made to assume any desired magnitude. t

The pre-suppression or reduction of amplification to an intermediate value provides the operating advantage in that before the shot instant, the gain of the amplier is reduced enough to eliminate the effect of many noises or electrical disturbances of relatively low amplitude. This means that the galvanometer trace will remain quiet because these low amplitude disturbances will not be amplied sufficiently to cause a material or bothersome movement of the trace. A quiet galvanometer trace increases the accuracy with which the rst break may be determined because it may be assumed with condence that the true rst break occurs at the very beginning of movement of the galvanometer trace. Moreover, the input signal applied to the amplifier 5 will be of large magnitude as compared with any noise or spurious disturbance of relatively low magnitude, thus producing a sharp and distinctive record of the rst break.

As explained in Shimek Patent No. 2,317,334, there may be provided a means for producing a voltage impulse coincidentally with the instant of explosion of the charge. This impulse, of substantial magnitude, corresponds with the instant of creation of the seismic Waves. As the dynamite comprising the shot is detonated, this impulse is produced and is immediately applied to the input circuit of the pentode 8 or directly to the recorder 1. Sometimes an uphole geophone, adjacent .the shot hole, may also be arranged to apply its output directly to the recorder, as shown in said Shimek patent. The additional details of these circuits have not been disclosed herein for the sake of simplicity.

Either the impulse indicative of the shot instant, or the arrival of the direct traveling waves at the uphole geophone, may be utilized as the initiating signal 40 of the master controller M shown in Fig. 1. The initiating signal is applied asV indicated at 40 in Fig. 1, to a transformer 4I and to the input circuit of a gas tube or grid-controlled rectifier 42. This gas tube 42 may be of the type known on theA market as a "Thyratron. It has a characteristic such that if the grid is negatively biased, as by the battery C, it will not conduct current. However, upon the application by the transformer 4| of a voltage which suiiciently reduces the negative bias (or applies a positive bias), the grid loses control and the tube 132 becomes conductive. When this occurs, current ows from the source of supply indicated by the battery i3 by way of the operating coil oi the relay 22, through the gas tube 42, a resistor Lili, and to the other side of the source of supply d3, The relay 22 is of the high speed type and immediately upon flow of current through the gas tube 42 the relay 22 opens its contact 2 l. The result of the opening of contact 2i is to transfer the connection of the screen grid of pentode i5 from the point 23 on the battery to the point c5, which transfer is in the direction for the supply of less voltage to the screen grid.

However, the screen grid voltage is not immediately reduced because there is included in the circuit a resistor it and a capacitor l. This parallel R-C combination produces a gradual, logarithmic decrease in the screen grid Voltage. The rate of decay of this voltage may be readily adjusted Iby varying the value of the resistor 46 or by movement of a selector switch 48 to connect one of additional capacitors t9 and 50, of differing size, in the R-C circuit combination.

After the recordation of the shot instant, it is desirable to record the instant of arrival of the direct traveling waves at the respective geophones. However, these Waves are of large amplitude and it is, therefore, necessary greatly to reduce the gain of the amplifier immediately after their arrival. 1t will be remembered that the current through the Thyratron 42 ilowed through the resistor ed. The potential difference or voltage across this resistor l is applied by conductors 52 and :i3 to another R-C combination comprising a variable resistor 5d and a capacitor 55. After a predetermined time interval, determined by the values of this R-C combination, the capacitor 55 is charged to reduce the negative b-ias applied by a bias battery C to the control grid of a gas tube or Thyratron El. Hence, immediately after arrival of the direct traveling waves at the geophone i0, the Thyratron 5l fires and current flows from the source or battery 43 by way of conductor 60, operating coil or" the relay 36, through the Thyratron 5i, and by way of conductor 53 to the other side of the battery.

The relay 3B is also a high speed relay which operates, immediately to transfer the connection of the control grid of the triode I6 to the Contact Si of the relay 35. The contact SI is connected by conductor 62 to a potentiometer 63 which greatly increases in a negative direction the bias between the grid and the anode of triode I6. This decreases its conductivity and hence increases the negative voltage Ede applied between the sup-- presser grid Sc and the cathode of pentode 8. The extentJ of the increase in this negative gaincontrolling bias is directly controlled by the setting of the potentiometer 53. 1t may be adjusted from zero to a maximum negative value adequate to render the triode I6 non-conductive. If the potentiometer 63 is set for a negative Voltage which does not make the tube non-conductive, the aforesaid compensating effect, (Eau-1R32) will stili be present; that is, signals applied to the control grid ci the pentode i5 will not affect the new value of the gain-suppressing bias Ede.

While the foregoing has ibeentaking place, it

will be remembered that the screen grid voltage of the pentode l5 has been decreasing due to the decay of the voltage across the R-C combination @f3-lil, The decrease in the screen grid voltage decreases the conductivity of the pentode l5. Consequently, the voltage Ese decreases and this, of course, reduces the current flowing through the resistance 32. As these voltages are reduced, a point is reached at which the triode :I6 becomes non-conductive; that is, for the value oi the negative bias on the grid, the diiierence in potential between the anode and cathode of triode i6 is insufcient for current ilow. Thereafter, the gainsuppressing bias Ede decreases with the decrease in potential or bias on the screen grid of the pentode I5. As the gain-controlling bias Ede decreases, from a maximum negative value to lesser negative values, the gain of the amplifier increases.

-As soon as the triode iii becomes non-conductive, the aforesaid compensating effect disappears. Therciore, during the subsequent increase of gain, the signals applied to the control grid oi the pentode i5 also control its output. This output is filtered by the 'dlt-er 35i- 32 and the resultant potential, of course, is applied to the suppressor grid 8c or" the pentode il. if the signals applied to the pentode i5 increase in amplitude the negative bias is correspondingly increased and vice versa. Thus the operation is similar to conventional automatic Volume control systems but diiers therefrom in that the pentode i5 serves as an automatic volume control during the rise in amplification oi the ampliner 5 which is due solely to the decrease in the screen grid voltage as controlled by the R-C combination of resistor #i5 and capacitor fil. in other words, the automatic volume control is eective around a constantly changing operating point, one in which the overall gain is increased exponentially with time7 or gain which over a time interval increases inversely as the attenuation of succeeding wave trains increases. Thus the automatic volume control corrects for the dierences between the actual attenuation cf Wave trains and the exponential rise in the overall ainplier gain. This has the advantage of greatly reducing the load or regulating duty of the automatic volume control feature.

The net effect is that the instantaneous corrections due to the automatic Volume control are so small as not to themselves introduce signale which would cause the Zero line of the seismogram to change, or to introduce a spurious signal into the seismogram.

Referring to Fig. 2, the gain of the ampliiier is indicated by the legend Maximum gain applied to the broken line iii which appears on a graph in which gain is plotted as ordinates against time as abscissae. At Zero time, the shot instant, it will be observed the amp-liner 5 will be operating with its gain reduced indicated by the horizontal line ll. Of course, its gain will have been reduced prior to zero time because Zero time is assumed to be the time at which the dynamite is detonated.

Upon detonation oi the dynamite, or upon arrival of seismic waves at an uphole geophone, the resultant initiating signal is applied. at to the transformer di, Fig. l. The Thyratron #i2 res and the relay 22 immediately opens its contact 2i to initiate the decay of the voltage applied to the screen grid of pentode i5. As this Voltage decreases, the plate current of the pentode l5 also decreases. This effect, in the absence of the other provisions of the invention, would cause an increase in the gain of the amplier 5 as indicated by the broken line 'l2 of Fig. 2. The actual gain of amplifier 5 remains at the high level as indicated by the horizontal line 'H until the time Ti. At this time, the Thyratron 51 fires and produces immediate operation of relay 36 to complete a circuit from the control grid of triode I6 for application thereto of a relatively high negative bias, the net effect of which is greatly to reduce the amplication of amplifier 5. This substantially instantaneous reduction occurs as indicated by the line '53. The new minimum or low level of amplication is shown by the horizontal line lll.

The substantially instantaneous increase in Ede, and corresponding decrease in amplification, is accomplished by having available a large voltage Esc at the instant Ti. This large voltage is used to drive the network, consisting of resistor 32 and capacitors 3G and 3l, at a rate such that the value of output voltage Ede, corresponding to the value of gain lf3 in Fig, 2, is reached quickly.

This is illustrated in Fig. 3, where successively larger constant values of Esc are assumed as being applied to the R-C network. In this illustration the tube or valve IG is assumed to have zero resistance until Zero time, and thereafter to have a value of resistance, as governed by its grid-cathode potential, such that Ede is constant.

The extensions of the voltage-time curves Ela@ to E43@ into the negative region below the level of Ede correspond with an innite value of resistance for the tube i0. As the voltage Eau is increased from Else to Ein. the gain-controlling voltage Ede is attained in progressively shorter times as shown at Td and Ta.

In the absence of the tube i6 the voltage Esc would have to be varied between the limits corresponding to the gain values 'H and 74 in Fig. 2. The rate of rise of the voltage Ede would be governed solely by the time constant of the R-C network. However, by utilizing the tube or valve I6 in accordance with the invention, the feature of substantially instantaneous reduction in gain at Ti is possible, regardless of the time-constant of the R-C network. The described action of the tube l makes possible the use of high driving voltages Elan to Etac to produce a fixed output voltage Ede.

.Again referring to Fig. 2, the time interval between and Ti is chosen long enough to allow for the first arrival of waves at all geophones on the spread, The gain of the amplifier 5 is then greatly reduced to the level indicated by the line 'ill in Fig. 2. During this time interval, from T1 to T2, the level of amplication is of a value such that direct traveling waves and shallow reflections of large intensity are recorded within the limits of the seismograin. The triode I6 becomes substantially non-conductive at time T2. Hence, the output from the pentode l5 is thereafter effective to control the increase in the gain of the amplifier 5 when signals of lower level are arriving.

As shown by the curve 72a, the eiect of the decreasing voltage on the screen grid is to produce an increase in gain. The amplification or gain rises from its minimum value at 14 to above the intermediate value at 'H and rises until it reaches its maximum value at l0. For reflections from the deeper strata the amplifier 5 will be operating at maximum gain.

It is again emphasized that from time T2 the output of the pentode i5 will not only depend upon the value of the screen grid voltage but also upon the intensity of the signals applied to the control grid. This automatic volume control feature is effective around a constantly changing amplifier gain; that is, the rise in amplification follows the gain curve '12a and as the gain rises the automatic volume control is effective around a constantly changing operating point, one which moves upwardly along the gain-increasing curve 12a.

Though there is nothing critical about the circuit constants, a number of values are given as exemplary of one form of the invention. The capacitors 3D and 3| may be 0.1 microfarad; capacitors 4l, 49 and 5D may be 4.0, 8.0 and 12.0 mfds. respectively, while resistors 32, 26 and 46 may loe 0.5 megohm, 0.1 megohm and 50,000 ohms respectively. Triodes of any type used in amplifiers will be satisfactory for the tube I6. The valve or tube I5 should be a sharp cut-off pentode, such as the 6J?.

The master controller is common to each amplier, there being provided on relays 22 and 36 additional circuit-controlling contacts which serve to control the circuits of each amplifier, as explained in connection with the amplifier 5. As in the case of amplier 5, the other amplifiers will each be provided with tubes l5 and i6 and circuits corresponding with those below the line which indicates the amplifier chassis in contrast with the vmaster controller chassis M above line 8l.

While a preferred embodiment of the invention has been described, it will be understood that further Imodifications may be made without departing from thespirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. The combination with an amplifier, of means for controlling the gain of said amplifier comprising a circ-uit for applying a gain-controlling bias to said amplier, said circuit including an. electric valve, a resistor connected across the output thereof, a second electric valve in shunt therewith, with a lter including a resistor connected therebetween, means for biasing said first valve for flow of current through its output resistor thereby to develop said gain-controlling bias, said second valve and said resistor of said lter cooperating to maintain at a predetermined Value said gain-controlling bias over relatively wide variations in output of said first electric valve, said rst valve including at least two control electrodes, means connecting one of said control electrodes to the output of said amplifier, means for controlling the biasing potential of the other of said control electrodes including a resistor and a capacitor, and means for rendering said second valve non-conductive thereby to transfer control of said gain-controlling bias to said first-named valve.

2. In a seismic prospecting system having means for producing electrical signals representative of seismic waves, an amplifier for said signals, and a recorder for recording said signals, the combination of an electric valve having an input circuit responsive to said signals and an output circuit including a load resistor, a lter having resistance and capacitance, a second electric valve in shunt with said filter to form a variable resistance, said second valve having a cathode, an anode and a control electrode, means including said filter for varying the bias between said cathode and said grid to change the resistance of said second valve so as to maintain. con- 9 stant the potential diiference produced across that valve, and means for applying said potential difference to said amplifier to control the gain thereof.

3. In a seismic prospecting system having means for producing electrical signals representative of seismic waves, an amplifier for said signals, and a recorder for recording said signals, the combination of an electric valve having an input circuit responsive to said signals and an output circuit including a shunting load resistor, a lter having a resistor in series with said output circuit and capacitors in shunt therewith, a second electric valve in` shunt with said lter to form a variable resistance thereacross, said second valve having a cathode, an anode and a control electrode, means including said lter for varying the bias between said cathode and grid thereby to vary the resistance of said second valve so as to maintain substantially constant the potential difference produced across that valve, and means for applying said potential difference to said amplier to control the gain thereof.

4. The combination with an amplifier, of a gain-controlling system comprising an electric valve having a control grid and a cathode connected in an input circuit responsive to signals from said amplifier and having its anode and cathode connected in an output circuit, a source of anode supply, a resistor connecting the positive side of said source t the anode of said valve, a second electric valve having its anode connected to said positive source of supply and to ground, means including a second resistor for connecting the cathode of said second valve to the anode of said first valve for producing a bias between the grid and cathode 0f said second valve to vary the conductivity and resistance of said second valve, and means for applying a bias derived from between said cathode and anode of said second valve to said amplier for controlling the gain thereof.

5. The combination set forth in claim 4 in which there are provided means for rendering said second valve substantially non-conductive to eliminate the effect of its variable resistance upon said amplifier.

6. The combination set forth in claim 4 in which there are provided means for rendering said second valve substantially non-conductive to eliminate the effect of its variable resistance upon said amplifier so that the bias thereafter applied to said amplier is largely dependent upon the Voltage across said output circuit, and

10 means operable as a function of time gradually to change the output of said first valve to Vary as a function of time the gain of said amplifier.

'7. The combination with an amplier, of a gain-controlling system comprising an electric valve having a control grid and a cathode connected in an input circuit responsive to signals from said amplifier and having its anode and cathode connected in an output circuit, a source of anode supply, a resistor connecting the positive side of said source to the anode of said valve, a second electric valve having its anode connected to said positive source of supply and to ground, means including a second resistor for connecting the cathode of said second valve to the anode of said rst valve, capacitive means, means connecting said capacitive means in shunt with respect to said second valve, means for applying to said amplifier the voltage drop across said second valve as a gain-controlling bias therefor, and means for initially rendering said second valve conductive to provide a uniform gain-controlling bias and thereafter rendering that valve non-conductive to apply a variable gain-controlling bias to said amplifier.

8. The combination set forth in claim 7 in which there is provided means for producing an initiating signal and in which said means for rendering said second valve non-conductive comprises means operable a predetermined time in terval after production of said initiating signal.

9. The combination with an amplifier of seismic signals of a gain-controlling system therefor comprising an electric valve having a control grid and a cathode connected in an input circuit responsive to seismic signals from said amplifier and having its anode and cathode connected in an output circuit, a source of anode supply, a resistor connecting the positive side of said source to the anode of said valve, means for producing over a predetermined time interval a gain-suppressing bias of substantially constant magnitude comprising a second electric valve having an anode connected to said positive source of supply and to ground, means including a second resistor for connecting the cathode of said second valve to the anode of said first valve, capacitors connected in shunt with said second valve, one on one side and one on the other side of said second resistor, and means for controlling the bias on said second-named valve to render it non-conductive after expiration of said predetermined time interval.

EDWIN J. SHIMEK. WILLIAM B. HEMPHILL. 

